Voltage regulator circuit



July 17, 1956 T. P. TlssoT, JR 2,755,377

VOLTAGE REGULATOR CIRCUIT Filed July 9, 1952 2 Sheets-Sheet l July 17, 1956 Filed July 9, 1952 T, P. TISSOT, JR

VOLTAGE REGULATOR CIRCUIT 2 Sheets-Sheet 2 200 fed/j a E1/40060 /50 f Z -,e. /4aow (cfm/vain [may /25 w #www5/w ar 74P /49 F 2 75 25 i0 75 /aa /25 /50 f7; 17.

f2 (Md) 56 e. f7 g en/if 7V ATTORNEY United tates Patent lhce 2,75 ,3 77 Patented July 17, 1956 VOLTAGE REGULATOR CIRCUIT Thomas P. Tissot, Jr., Haddoniield, N. J., assignor to Radio Corporation of America, a corporation of Delaware This invention relates to a voltage regulator circuit, and more particularly to a regulator circuit useful in the screen grid power supply for tetrode or pentode vacuum tubes.

Tetrod'es or pentodes are ordinarily used in one or more stages of a television transmitter, including the modulator stage thereof, for the purpose of obtaining high power with maximum eiciency. ln such a transmitter, it is generally necessary to regulate the screen grid voltage of the modulator stage, as well as that of other stages, to accurately establish the pedestal level and synchronizing pulse peak level, independently of picture content. When the anode voltage of the tetrode or pentode falls to a low value and the screen grid voltage remains substantial'y constant, the screen current will rise, since the electrons emitted from the cathode are then no longer drawn through the screen to the anode but instead travel to the screen itself. lf the screen grid voltage is well regulated (that is, if it does remain constant) the input power to the screen grid will be excessive under conditions of low anode voltage. T his excessive screen grid power may darnage the corresponding tube or tubes.

Therefore, an object of this invention is t'o devise a screen` grid power control circuit which operates to prevent ex'cessive' input power from being applied to the screen grids of tetrode or pentode tubes when the anode voltage of such tubes drops to a low value.

Another object is to devise a regulated screen grid power supply which functions, under excessive load current conditions, to decrease the available output voltage.

A further object is to devise a regulated screen grid power supply which automatically becomes unregulated under conditions of excessive loadv current.

A still further object is to devise a novel screen grid power supply which effects a protective' action under certain conditions.

For purposes ef illustration, the invention will be described as applied to a cathode modulation arrangement wherein the' anodecathode paths of the modulator tubes are connectedin series with the anode-cathode path of the radio frequency (R. F.) power amplifier tube to be modulated; In such an arrangement, there exists the possibility of the anode voltage of the modulator stage being low under some condition of misadjustment. Under these conditions, the modulator tube or tubes must be prevented from possible damage which would result from excessive screen grid input power. Howeven, the invention is equally applicable to any circuit arrangement whereinJ a regulated voltage must be fed to a screen" grid' and wherein'- protectionv is needed when the anode voltage remains low for extended periods.

The objectsof this invention are accomplishembrieily, in the following manner: A series regulator tube has! its .emule-cathodel path' connected in series between an unL regulated power supplyl source andthe screen grid ciectrodes of one or more modulator. stages. A two-stage direct-current amplifier has-its input coupled Vto the oathode of the regulator tube and its output coupled tothe control grid of the regulator tube. in such a way as to tend to keep the voltage at the load terminal constant as the current drawn by the load (screen grid of one or more tetrodes or pentodes) changes. A resistor (for convenience at least initially adjustable) is connected in series between` the regulator tube anode and the unregulated power supply source, of such value as to drop the voltage at such anode, to thereby cause the load terminal voit'age to drop, as the current drawn by the load (screen grids) tends to become excessive.

The' foregoing as well as other objects of the invention, will be best'un'dersto'od from the following description of an exempliication thereof, reference' being had to the accompanying drawings, wherein:

Fig. l is a simplied schematic of a circuit arrangement according to this invention;

Fig. 2 is a set of curves illustrating the operation of the Pig'. 1 circuit; and

Figs. 3a and 3b taken together are a complete schematic of a practical embodiment of Fig. l, such as might be used in a television transmitter.

Now referring to Fig. l, screen grid power for the screen grids oftransmitter tubes to' be protected (which are not shown in Fig. l', but whichk may be modulator stage tubes in the transmitter) is derived from an unregulated screen grid power supply the negative output lead of which is grounded (connected to a point of zero reference potential) andV the positiver output lead of which is connected through a variable resistor R to the anode 2 of a series regulator triode vacuum tube 3. The purpose and action of resistor R will hereinafter become apparent. The cathode 4 of tube 3 is directly connected to the high potential; output terminal 5 and the load voltage (herein designated' at En) or output voltage of the circuit appears between output terminals 5 and 6, terminal 6 being grounded. According to this invention, as an illustration, th'e load voltage EL may be utilized for the screen voltage of a screen-grid-type tube or tubes, such as tetrodes or pentodes. Since the anode-cathode path of tube 3 and resistor R are both in series between source 1 and the load (which is connected across output terminals 5 and 6), the load current IL liows through resistor R and the tube` 3.

A two-stage D. C. amplifier 7 has its input connected to cathode 4 of the regulator tube and its Voutput connected to the control grid 3 of this tube. The amplifier 7'" consists of a twin triode vacuum tube 9 and its circuit connections. The input to this amplifier is taken from the regulator tube output terminal 5 through a resistor 10 connected between this terminal and the right-hand cathode 1:1 of tube 9. Cathode 11 is connected' to ground through a resistor 12. A substantially constant bias is provided on theY right-hand control grid 13 of tube 9 by means' of a potentiometric tap 14 connected to said grid and movable on the second one 16 of two series-connected resistors lil and 16 arranged between the left-hand cathode 17 of tube 9 and ground. The voltage across the series combination 15S-i6 is maintained constant by means of a gaseous voltage regulator tube 18.

The tube 9 is arranged as a two-stage D. C. amplifier. An anode load resistor 19 is connected between the righthand anode 20' of tube 9 and terminal 5. Anode 20 is directlyV coupled to the left-hand control grid 21 of tube 9, while the output of D. C. amplifier 7 is taken oil" by means of a connection between the left-hand anode 22 of tube 9 and grid 8 of tube 3.

The D. C. amplifier 7 operates, in conjunction with regulator tube 3, to maintain the load voltage EL conslant over a certain range of load current Ir., when the circuit is in normal operating condition. Any incipient change of loadvoltage EL (las long as the load current Ir. stays below a fixed predetermined level) is appliedI to the cathode 11 input of D. C. amplifier 7. The bias voltage on control grid 13 is held substantially xed by means of a gaseous voltage regulator tube 18 connected across resistor network -16, the lower electrode of tube 18 being connected to ground and the upper electrode of this tube being connected through a resistor 23 to the positive supply lead and anode 2. The voltage change across the first stage anode load resistor 19 is applied to the grid 21 of the second stage amplifier, and the voltage change across the second stage anode load resistor 24 (connected between anode 22 and the positive supply lead to anode 2) is applied to the grid 8 of regulator tube 3. The connections are such that if the voltage of output terminal 5 tends to increase, the voltage applied to grid 8 by way of amplifier 7 will be of such a polarity as to decrease this terminal voltage, while if the voltage of terminal 5 tends to decrease, the voltage applied to grid 8 by way of amplifier 7 will be of such a polarity as to increase this terminal voltage.l Thus, the series regulator tube 3 operates in conjunction with the D. C. amplifier 7 to maintain the output voltage at terminal 5 quite constant, at least over a range of load current Ir. below a fixed predetermined level. In other words, as the load current Ir. varies below this fixed level, the control tube 9 voperating as amplifier'7 adjusts the grid voltage on tube 3 to maintain a fixed terminal voltage EL; thus, the voltage EL is independent of load current IL within this range of load current.

According to this invention, the series resistor R performs a very important regulating function in the circuit which has been described. It has previously been stated that the load current IL flows through the series resistor R, as well as through tube 3. As a typical example, the resistor R may be set at approximately 1200 ohms. As the load current Ir. increases, the control tube 9 adjusts the voltage on regulator tube grid S to maintain a fixed terminal voltage EL. However, the voltage drop across resistor R increases with increasing IL, since IL fiows through R. Therefore, the anode voltage of tube 3 is caused to decrease as IL increases. At the same time, the voltage on grid 8 is being increased by the action of the regulating amplifier 7, to tend to counteract the decrease of voltage at cathode 4 and to tend to keep the terminal or output voltage EL constant. j When the grid potential (which is being increased) appreaches the anode potential (which is being decreased) the regulator tube 3 begins to function as a diode and as a result the current passed thereby is no longer dependent upon the grid potential. It should be readily apparent that when a triode is operating as a diode, the grid thereof is no longer able to exercise any control. Therefore, when this point is reached (at a fixed predetermined value of IL sufiicient to drop the voltage at anode 2 suciently to cause tube 3 to operate as a diode) the power supply constituted by source 1 and the regulator tube 3 becomes unregulated and the output voltage EL decreases with increasing load current Ir. (which due to the volt-age drop in resistor R causes the voltage at anode 2 to decrease). Then, the output power available from the aforesaid power supply is essentially constant.

In other words, the resistor R acts to cause the terminal voltage EL to fall rapidly after a certain (predetermined) load current IL has been reached. After this point is reached, the product of EL and IL remains substantially constant (even as In continues to increase) and the input power to the modulator tube screen grids (which, as stated, may be connected to output terminal 5) remains substantially fixed. Limited regulation thus becomes effective at a predetermined value of load current IL, .preventing the input power to the screen grids of the tetrodes or pentodes from becoming excessive and preventing any possible damage to the tubes the screen voltage of which is regulated.

Since, in effect, the terminal voltage EL is regulated only over a certain range of load current IL below a predetermined value thereof and is essentially unregulated above this value of load current, the power supply comprising source I and regulator tube 3 may be termed a limited regulation supply; the notation limitedly regul lated has therefore been applied t0 EL in Fig. l. If this limited regulation operation did not take place due to the action of resistor R, that is, if the terminal voltage EL were caused to remain constant throughout the entire range of variation of load current IL, the available output power (product of EL and IL) would be excessive when IL increases, and thus the input power to the screen grid load would be excessive, resulting in damage to the screen grid tubes.

Fig. 2 is a representative set of curves illustrating the operation of the circuit of Fig. 1. These curves represent the relation between IL and EL, for the various values of R indicated, the lower curve being for the same value of R as the middle curve, but with a different adjustment of tap 14 on resistor 16. Up to the point (a predetermined value of load current IL for each curve) where only limited regulation goes into effect, the voltage EL is independent of load current IL, as indicated by the horizontal portion of each curve. Beyond these points, the resistor R causes the available output voltage Er. in each case to fall rapidly, as represented by the downward direction of each curve.

When the anode voltage of screen grid tubes drops to a low value, the screen current rises. However, with this invention, when a certain load current (screen current) IL has been reached, the screen voltage EL drops (as illustrated in Fig. 2), preventing excessive input power from being applied to the screen grids. The screen grid power supply of this invention automatically becomes unregulated Linder conditions of excessive load current IL, since after a certain load current is reached the terminal or output voltage EL falls ofi rapidly.

Fig. 3 is a schematic circuit diagram of a practical embodiment of this invention, as applied to an actual television transmitter which was built and successfully tested. This transmitter was designed to operate in the U. H. F. television band (470-890 megacycles) with a power output of l kilowatt. In Fig. 3, parts the same as those of Fig. l are indicated by the same reference numerals.

In the practical embodiment of Fig. 3, the screen grid unregulated power supply 1 furnishes 350 volts D. C. to resistor R, and the series regulator vacuum tube 3 is a twin-triode type such as the 6AS7-G, with the electrodes of its twin triode units respectively connected essentially in parallel. In place of the voltage-regulator resistor arrangement 18, 23 in Fig. l for maintaining a constant voltage across resistors 15 and 16 (and therefore a fixed but adjustable voltage on grid 13), in Fig. 3 the common junction of cathode 17 and resistor 1S is connected to the 75-volt terminal of a regulated power supply (not shown). In Fig. 3, a capacitor 25 is connected from cathodes 4 to ground. The extra resistors associated with tube 3 in Fig. 3 (i. e., the resistors between anodes 2 and that end of resistorR remote from power supply 1, and the resistors between grids S and anode 22) are used mainly for parasitic-suppression purposes` The vacuum tube 9 may be a twin-triode type 6SL7-GT. The anodes of tubes 26 obtain their positive polarizing or operating voltages through tube 35 and such anode operating voltage is greater than the operating voltage applied to the screen grid electrodes of tubes 26.

In Fig` 3, the variable resistor R may have a maximum value of 1500 ohms, being adjusted to provide an effective value in the circuit of approximately 1200 ohms, for proper operation.

The screen grids of the modulator tubes to be protected are connected to output terminal 5 of the regulated screen grid power supply, which terminal is connected to cathodes 4 of the series regulator tube 3 as in Fig. 1. Thus, the modulator stage may include eight vacuum tube pentodes 26, for example of the 6146 type, connected in parallel. The screen grid 27 of each of tubes 26 is connected through a separate respective resistor 28.to terminal 5. The total screen current drawn by all of modulator tubes 26 is thus derived from the regulated screen grid power supply of this invention and the entire screen current Hows through tube 3 and resistor R. The total screen current drawn by all of tubes 26 therefore corresponds to the load current IL in Fig. l, While the voltage applied to screens 27 corresponds to the load voltage EL in Fig. 1.

The video signal from the iinal video amplifier stage (not shown) is coupled through a capacitor 29 to a common lead 30, from which separate respective resistors 31 extend to the control grid 32 of each of the tubes Z6. 1n this way, the video signal is applied essentially in parallel to the control grids of all the modulator stage tubes 26.

The output of a keyed clamp circuit (not shown) is applied to the lead 30 in order to periodically clamp the control grids 32 of the modulator tubes 26 at a predetermined level, to thereby reinsert or restore the D. C. video component which may have been lost due to A. C. coupling between prior video amplifier stages. A typical clamp circuit which rnay be used for this purpose is disclosed in the copending Gluyas application, Serial No. 300,480,

led July 23, 1952.

An F.. F. driver 33, which may include a plurality of amplifier and frequency multiplier stages, supplies R. F. carrier energy to the control grid 34 of a power tetrode vacuum tube 35 constituting a R. F. power amplifier stage, through a tuned or resonant input circuit 36. Although input circuit 36 is schematically illustrated as a lumpedconstant circuit, it actually i-s a cavity resonator type of circuit. Tube 35 may be of type 6181.

In order to amplitude modulate the carrier supplied to tube 35 by means of the video signal supplied to control grids 32, cathode modulation is employed. That is to say, the anodes 37 of all the modulator tubes 26 are directly connected to the cathode 38 of the R. F. power amplifier tube 35, while the anode 39 of tube 35 is connected to the positive terminal of a high voltage unidirectional source through a resistor 40 and the winding of a tuned output circuit 41 (which circuit may also be of the cavity resonator type). To complete the modulation circuit, the cathode 42 of each of the tubes 26 is connected through a separate respective resistor 43 to a common lead 44, from whence a series-connected choke 45 and a resistor 46 are connected to ground. Amplitude modulation of the R. F. carrier supplied to tube 35 is thus etected by means of the cathode modulation circuit described, and amplitude modulated R. F. output is taken off and fed to a suitable load (such as an antenna) by means of a coupling arrangement suitably coupled to output circuit 41.

In order to improve the linearity of the transmitter modulation characteristic, video currents may be taken ot (as indicated) from cathode lead 44 and fed back to a prior video amplifier stage. The cathode modulation circuit described, including video feedback, is disclosed in the copending Gluyas application, Serial No. 294,215, tiled June 18, 1952, now Patent No. 2,728,892, dated December 27, 1955.

In Fig. 3, the resistor R operates, in conjunction with the tube 3 (in exactly the same manner as in Fig. 1), to cause the voltage at terminal 5 (screen voltage for the modulator tubes 26) to drop rapidly when a predetermined value of CII 6 load current (screen current of the modulator tubes 26) is reached and passed. The mode of operation is the same as in Fig. 1, so the description thereof will not be repeated.

What is claimed is:

1. In a regulated power supply circuit, a source of unregulated unidirectional voltage, output terminals to which a variable-current load may be connected, an electron discharge device having an anode, a cathode, and a control electrode; means connecting the anode-cathode path of said device in series between said source and said terminals, means connected to said output terminals and operating automatically in response to changes in the voltage across said terminals for applying a variable voltage to said control electrode, said last-named means including a resistor connected between said anode and said control electrode; and a resistor connected between said source and said anode in series with said anode-cathode path, said last-mentioned resistor having an elective resistance such that currents above a predeterminedV value owing therethrough cause a voltage drop thereacross suticient to make the anode voltage of said device approach the control electrode voltage thereof, thereby to cause said device to operate substantially as a diode and to substantially eliminate the effect of said variable voltage on said device.

2. In a regulated power supply circuit, a source of unregulated unidirectional voltage, output terminals to which a variable-current load may be connected, a voltage divider resistor connected across said terminals, an electron discharge device having an anode, a cathode, and a control electrode; means connecting the anode-cathode path of said device in series between said source and said voltage divider resistor, means for controlling the impedance of said device 'in accordance with changes in the voltage across said voltage divider resistor, said last-named means comprising a direct current amplifier having an input circuit connected to said voltage divider resistor and an output circuit connected to said control electrode, the last stage of said amplifier being energized through a resistor connected between said anode and said control electrode; and a resistor connected between said source and said anode in series with said anode-cathode path, said last-mentioned resistor having an elective resistance such that currents above a predetermined value owng therethrough cause a voltage drop thereacross suicient to make the anode voltage of said device approach the control electrode voltage thereof, thereby to cause said device to operate substantially as a diode and to substantially eliminate the eiect of said variable voltage on said device.

References Cited in the tile of this patent UNITED STATES PATENTS 2,210,394 Braden Aug. 6, 1940 2,355,191 Vance Aug. 8, 1944 2,424,905 Scheldorf July 29, 1947 2,462,935 Baker Mar. 1, 1949 2,572,832 Bernard Oct. 30, 1951 2,624,869 Knoebel Jan. 6, 1953 

